Radical polymerization is a well-known polymerization technique. The use of alkoxyamines makes it possible to prepare block copolymers.
These polymerizations are almost always performed at temperatures at least higher than 100° C. An exception concerns the use of N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, also known as SG1, since it makes it possible to start the polymerization at 50° C. This nitroxide, the formula of which is recalled below, is more particularly described in EP 0 760 824:

Although this nitroxide has many advantages such as moderate polymerization temperatures and good control of many monomers, it does not allow polymerizations to be performed at low temperature such as room temperature (for instance 20° C.) or even below room temperature within times that are compatible with industrial cycles when it is derived from alkoxyamines, since the latter can only generate the persistent radical at temperatures above 50° C.
WO 03/074 572 describes the possibility of polymerizing in a controlled manner via the nitroxide route at temperatures of between 50° C. and 160° C., but preferably between 80° C. and 100° C. The reaction is thus not possible, for example, at room temperature.
Now, there is a real need to be able to perform polymerizations at such low temperatures, or even at negative temperatures, for example in the case of the polymerization of very large components (for example wind turbine blades or boat hulls) for which a heating installation of oven type is very expensive. A polymerization performed at room temperature or even at negative temperatures would thus be a major economical advance or would make it possible, for example in the case of multipack adhesives, for them to be usable under severe negative temperature conditions.
Moreover, in a formulation comprising block copolymers, the introduction of block copolymers synthesized in a preliminary step usually gives rise to a very significant increase in the viscosity of the mixture, which constitutes a major obstacle for processes requiring a low viscosity, such as infusion processes for the manufacture of composite materials, or high production-speed processes for which the rheological behavior of the preparation is crucial. It therefore appears judicious to prepare the block copolymer in situ, i.e. during the forming of the material, and to do so at low temperature (>0° C.), typically at room temperature, i.e. at about 25° C.
In other situations, low-temperature polymerization has advantages (limitation of side reactions, no degradation of certain heat-sensitive molecules), but also makes it possible to consume less energy or to minimize the pressure in the synthesis reactors.
In standard radical polymerization, it is well known, when the initiator used is a peroxide, that the addition of amines such as dimethyl-para-toluidine (Qiu K. et al., Polymer Communications, No. 1, 76-81, 1985) allows initiation at low temperature. This redox system is useful for preparing two-pack adhesives that can be used at room temperature, for example.
The Applicant has now discovered, contrary to all expectation, that an alkoxyamine, in the presence of a photoinitiator, re-initiates the polymerization of monomers under irradiation with electromagnetic radiation, and does so at negative temperatures, with rapid kinetics, i.e. with times that are compatible with industrial applications, typically from a few minutes to a few hours.